Steer-by-wire tiller with position feel system

ABSTRACT

A system and a method for generating a signal to control a steering mechanism and an aircraft using the system are disclosed. A tiller module includes a moveable tiller base configured to moveably respond to a steering signal. The tiller module also includes a tiller coupling configured to couple the tiller with the moveable tiller base and further configured to generate a tiller differential signal indicative of a steering force applied to the tiller relative to a position of the moveable tiller base. A tiller controller is configured to receive the tiller differential signal and a steering mechanism position signal. The tiller controller is further configured to generate the steering signal to direct a steering mechanism to conform with the steering force and to direct the moveable tiller base of each tiller module to a feedback position representative of a current position of the steering mechanism.

FIELD OF THE INVENTION

This invention relates generally to aircraft control and, morespecifically, to control of steerable aircraft landing gear.

BACKGROUND OF THE INVENTION

Steer-by-wire systems can greatly simplify design and sometimes,operation of moving vehicles. Designing a mechanically-steered vehicletypically involves positioning operator controls close enough tosteering devices to permit the operator controls to engage turnablewheels or steerable nose gear. To take the example of an automobile, aconventional rack and pinion steering mechanism involves placing thesteering column such that one end is directly coupled to the frontwheels and an opposite end is positioned in front of a driver's seat.Once the steering column is in place, other drive train and supportingcomponents are positioned around the steering column under the hood, andthe dashboard and other operator controls are positioned around thesteering column in the passenger compartment Therefore, significantdesign aspects of the automobile are dictated by a functionally-dictatedplacement of the steering controls.

Similarly, placement of mechanical operator controls for a nose gearsteering mechanism for an aircraft also dictates aspects of the designof the aircraft and flight deck. A tiller serving as an operator controlfor the nose gear steering mechanism is placed to enable practicalaccess to nose gear steering hydraulics. Design limitations are evenmore restrictive when considering the duplicate operator controls whichcommonly are used in aircraft flight deck. For example, providing twotillers to control a nose gear steering mechanism not only involvesplacing a first tiller with a practical mechanical linkage to the nosegear steering mechanism, but also involves placing a second tiller whereit can be mechanically linked with the nose gear steering system or withthe first tiller. Although the second tiller does not have to be coupleddirectly with the nose gear steering system, having a mechanical linkagebetween the two tillers may limit how other devices can be installed inthe cockpit around and between the tillers.

Using steer-by-wire controls to operate the nose gear steering systemcan alleviate some of these placement and design concerns. In asteer-by-wire system, a mechanical linkage between operator controls andthe steering device is replaced with an electrical system, therebyobviating the need for a direct mechanical linkage between the operatorcontrols and the steering device. More specifically, the operatorcontrols are linked with an electrical transducer that reads theoperator input and generates an electrical signal representative of theoperator input. The representative electrical signal is communicated toa corresponding electrical driver coupled with the steering mechanism,and the electrical driver directs the steering mechanism in a directioncorresponding to the operator input. Accordingly, the steering mechanismis controlled by the operator as he or she might control it using amechanical linkage but without there having to be mechanical linkagesbetween the operator controls and the steering mechanism.

However, steer-by-wire systems present concerns not shared in systemsemploying mechanical linkages. For example, steer-by-wire systems do notnaturally present feedback to an operator in the way thatmechanically-linked systems do. Once more considering the example of anautomobile, a mechanical linkage presents steering resistance to theoperator in proportion to the resistance caused by the sharpness of theturn. Also, while a mechanically-linked steering wheel naturally spinsback to normal as the steered wheels return to a straight-aheadposition, there is no such natural response imparted to operatorcontrols in a steer-by-wire system. Returning the steer-by-wire controlsto an original position involves an application of an impetus to returnthe controls to such a neutral position.

The example of nose-wheel steering in an aircraft having duplicatecontrols presents an additional concern. In a steer-by-wire system, itis possible that the pilot and co-pilot might both be trying to controlthe nose-wheel steering system, and the force being applied by oneoperator might not be communicated to the other operator through thecontrols. Accordingly, the operators conceivably might be applyingcontradictory inputs to the control devices without the other operatorknowing of it, defeating both their purposes. In an extreme case, bothoperators might apply equal force to the control devices therebyresulting in an excessive net steering input. Also, if such steeringproblems occur, and one operator suddenly releases his or her control,the response of the steering system to the unopposed or unsupportedforce on the control still being maintained could result inunder-steering or over-steering.

Thus, there is an unmet need in the art for a steer-by-wire system thatprovides feedback through the operator controls reflecting a status ofthe steering mechanism. There also is an unmet need for a steer-by-wiresystem for providing non-mechanical linking between multiple operatorcontrols such that each of the operators can be apprised of the effecton the steering mechanism resulting from other operators' actions.

SUMMARY OF THE INVENTION

The present invention provides a steer-by-wire steering system andmethod providing feedback to one or more operator controls indicative ofa position of the steering mechanism, and an aircraft using the system.Embodiments of the present invention use a tiller module in which atiller is coupled with a moveable base. The tiller is movably secured tothe base so as to be moveable within a range of a few degrees relativeto the base or is rigidly secured to the base. Force applied to thetiller by an operator is determined by measuring the relativedisplacement of the tiller relative to the moveable base or by measuringstrain in the tiller relative to the base. A differential signal ismeasured and applied to a tiller controller which generates a steeringsignal to the steering mechanism. A steering mechanism monitoringposition device communicates a position signal back to the tiller modulecausing the moveable base to be moved to reflect a position of thesteering mechanism. When multiple tiller modules are used, differentialsignals measured at each tiller module are summed and provided to thetiller controller. A steering mechanism monitoring position devicecommunicates the position signal back to each of the tiller modulescausing the moveable bases to be moved such that each tiller reflects aposition of the steering mechanism.

More particularly, embodiments of the present invention provide a systemand a method for generating a signal to control a steering mechanism. Atiller module includes a moveable tiller base configured to moveablyrespond to a steering signal. The tiller module also includes a tillercoupling configured to couple the tiller with the moveable tiller baseand further configured to generate a tiller differential signalindicative of a steering force applied to the tiller relative to aposition of the moveable tiller base. A tiller controller is configuredto receive the tiller differential signal and a steering mechanismposition signal. The tiller controller is further configured to generatethe steering signal to direct a steering mechanism to conform with thesteering force and to direct the moveable tiller base to a feedbackposition representative of a current position of the steering mechanism.

In accordance with further aspects of the present invention, the tillercoupling includes a centering spring mechanism configured to receive thetiller and allow movement of the tiller relative to the moveable tillerbase within a predetermined displacement range. The tiller inputcoupling includes a tiller position transducer configured to measure adisplacement of the tiller and generate the tiller differential signal.Alternatively, the tiller input coupling also includes a forcetransducer configured to generate the filler differential signal as afunction of a strain resulting from the steering force applied to thetiller relative to the position of the moveable tiller base. Inembodiments of the present invention, the tiller includes a rotatablehandle and the moveable tiller base includes a rotatable base moveableby a motor receiving the steering signal.

In accordance with further aspects of the present invention, the tillercontroller converts the tiller differential signal into the steeringsignal such that the moveable tiller base of each tiller module is movedto cause the tiller to be aligned to a position representative of thesteering mechanism position signal. The tiller controller integrates thetiller differential signal such that a rate of change of the steeringsignal is proportional to a magnitude of the steering force. The tillercontroller further includes a steering signal decay function such that areduction in the tiller differential signal causes a resulting reductionin the steering signal causing the steering mechanism and the moveabletiller base to return toward a neutral position. The tiller controllerfurther includes a position limiter restricting the tiller such that thefeedback position of the tiller is representative of the currentposition of the steering mechanism.

In accordance with still further aspects of the present invention,redundant components are used to allow the steering system to continueto operate even when some components have failed. Redundant secondarycomponents suitably are configured to perform the same functions asprimary components. The secondary components can quickly replace thefunction of failed primary components allowing the steer-by-wire systemto continue to function. In addition, a steering system using multipletiller modules provides fault tolerance in that, should one tillermodule fail, steering can still be controlled using the other tillermodules.

In accordance with further aspects of the present invention, the tillermodule includes a first tiller module having a first moveable tillerbase configured to receive a first tiller that is configured to receivea first steering force and a second tiller module having a secondmoveable tiller base configured to receive a second tiller that isconfigured to receive a second steering force. The first tiller modulegenerates a first tiller differential signal and the second tillermodule generates a second tiller differential signal. The tillercontroller sums the first tiller differential signal and the secondtiller differential signal such that the steering signal is generated todirect the steering mechanism to conform with a composite of the firststeering force and the second steering force. The steering signal causesboth the first moveable tiller base and the second moveable tiller baseto move to a feedback position representative of the current position ofthe steering mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 is a block diagram of a steer-by-wire system in accordance withan embodiment of the present invention;

FIG. 2 is a block diagram of a tiller module in accordance with anembodiment of the present invention;

FIG. 3 is a block diagram of a tiller controller in accordance with anembodiment of the present invention;

FIG. 4A is a top cutaway view of a flight deck of an aircraft equippedwith an embodiment of the present invention;

FIG. 4B is another top cutaway view of a flight deck of FIG. 4A;

FIG. 5 is a side view of an aircraft equipped with an embodiment of thepresent invention;

FIG. 6 is a flowchart of a routine using an embodiment of the presentinvention operating with a single tiller module; and

FIG. 7 is a flowchart of a routine using an embodiment of the presentinvention operating with two tiller modules.

DETAILED DESCRIPTION OF THE INVENTION

By way of overview, embodiments of the present invention provide asystem and a method for generating a signal to control a steeringmechanism. A tiller module includes a moveable tiller base configured tomoveably respond to a steering signal. The tiller module also includes atiller coupling configured to couple the tiller with the moveable tillerbase and further configured to generate a tiller differential signalindicative of a steering force applied to the tiller relative to aposition of the moveable tiller base. A tiller controller is configuredto receive the tiller differential signal and a steering mechanismposition signal. The tiller controller is further configured to generatethe steering signal to direct a steering mechanism to conform with thesteering force and to direct the moveable tiller base of each tillermodule to a feedback position representative of a current position ofthe steering mechanism.

FIG. 1 is a block diagram of a steer-by-wire system 100 for an aircraft(not shown) in accordance with an embodiment of the present invention.The steer-by-wire system 100 controls steering of nose gear 184. Thesteer-by-wire system 100 incorporates two tiller modules 110 a and 110b, a tiller controller 140, a nose gear steering actuation system 182, anose gear position sensor 186, and two summers 170 and 180. In theconfiguration of FIG. 1, the steer-by-wire system 100 provides dualtiller modules 110 a and 110 b so that, for example, a captain and afirst officer each can have his or her own tiller for steering the nosegear 184 of the aircraft. The steer-by-wire system 100 will be inexplained in more detail below and with regard to FIGS. 2 and 3.

Although the steer-by-wire system 100 is configured for aircraftsteering, it will be appreciated that the steer-by-wire system 100 isadaptable for other systems in which a steer-by-wire system 100affording advantages of the present invention are desired. For example,embodiments of the present invention could be used for land orwater-based conveyances, heavy machinery, cranes, or other devices. Thetiller could be in the form of a steering wheel, a yoke, a lever, oranother operator control device. Also, although FIG. 1 shows a system100 employing dual tiller modules 110 a and 110 b to take advantage ofelectrically linked feedback unifying operation of the dual tillermodules 110 a and 110 b, the present invention offers a steeringposition feedback system which provides advantages in asingle-tiller-system as well as for multiple-tiller systems.

More specifically in the dual-tiller steer-by-wire system 100, each ofthe tiller modules 110 a and 110 b receives a tiller 112 a and 112 b ina moveable base 114 a and 114 b, respectively. When an operator appliesforce to the tiller 112 a and 112 b, tiller sensors 118 a and 118 bgenerate differential output signals 130 a and 130 b. The differentialoutput signals 130 a and 130 b are received by the tiller controller140. The differential output signals 130 a and 130 b are added at asummer 142.

The tiller controller 140 processes the tiller differential signals 130a and 130 b as will be further described in connection with FIG. 3. Inaddition to the tiller differential signals 130 a and 130 b, the tillercontroller 140 also receives a nose gear position signal 188 from a nosegear position sensor 186 coupled with the nose gear 184. Using thetiller differential signals 130 a and 130 b and the nose gear positionsignal 188, the tiller controller generates a steering signal 160. Thesteering signal 160 represents a desired position of the nose gear 184.At a summer 170, the steering signal 160 can be added to an output of asecondary steering system 171 if such a secondary steering system 171 isdesired to supplement the tillers 112 a and 112 b. Assuming no suchsecondary steering system 171 augments the steering signal 160, thesteering signal 160 is fed to the summer 180. At the summer 180, thenose gear position signal 188 is subtracted from the steering signal160. An output 181 of the summer 180 is input to the nose gear steeringactuation system 182. The nose gear steering actuation system 182 steersthe nose gear 184 according to the magnitude and sign of the output 181of the summer 180.

Thus, when the steering signal 160 exceeds the nose gear position signal188, the output 181 generated by the summer 180 is positive. A positiveoutput 181 from the summer 180, that is in turn input to the nose gearactuation system 182, causes the nose gear 184 to be steered in what isdefined as a positive direction toward the desired steering positionindicated by the steering signal 160. A positive direction, for example,may be defined as clockwise or counterclockwise according to theperspective of FIG. 1. On the other hand, when the nose gear positionsignal 188 exceeds the steering signal 160, the output 181 generated bythe summer 180 is negative. A negative output 181 from the summer 180input to the nose gear actuation system 182 steers the nose gear 184 inwhat is defined as a negative direction toward the desired steeringposition indicated by the steering signal 160. When the steering signal160 and the nose gear position signal 188 are equal, the signals 160 and188 negate each other at the summer ISO, thereby resulting in a zerooutput 181 from the summer 180. A zero output 181 from the summer 180means a zero signal is applied to the nose gear steering actuationsystem 182, thereby causing the nose gear 184 to remain at its currentposition.

As the steering signal 160 is input to the nose gear steering actuationsystem 182, the steering signal 160 is also supplied to the tillermodules 110 a and 110 b. In response to the steering signal 160, themoveable bases 114 a and 114 b of the tiller modules 110 a and 110 b arerotated to reflect a position of the nose gear wheels 184. Rotating themoveable bases 114 a and 114 b in turn moves the tillers 112 a and 112b. As a result, the steer-by-wire system 100 provides visible andtactile feedback to the tillers 112 a and 112 b thereby reflectingposition of the nose gear 184. Moreover, in a multiple-tillerenvironment, like the dual-tiller 112 a and 112 b configuration of thesteer-by-wire system 100 of FIG. 1, the tillers 112 a and 112 b areelectrically linked to the nose gear 184 and each other.

Advantageously, visual and/or tactile feedback is provided to thetillers 112 a and 112 b without a mechanical linkage between the tillers112 a and 112 b and the nose gear 184 or the nose gear steeringactuation system 182. Such feedback is available and helpful to anoperator of a single-tiller system or a multiple-tiller system. In thecase of a multiple-tiller system, if a first operator directs his or hertiller 112 a to steer the nose gear 184 to a new position, a secondoperator using his or her tiller 112 b will have visible and/or tactilefeedback of the position of the nose gear 184. This linked feedback canavoid a problem of the operators applying opposing control forces to thetillers 112 a and 112 b or over-steering as a result of an operator nothaving feedback as to a position of the nose gear or the actions beingundertaken by another operator.

FIG. 2 shows an enlarged view of the structure of a tiller module 110,like tiller modules 110 a and 110 b (FIG. 1). The tiller sensor 118generates a differential output signal that is communicated to thetiller controller 140 (FIG. 1). In the embodiment shown in FIG. 2, thetiller 112 is movably mounted in the moveable base 114 such that thetiller 112 can be rotationally moved relative to the moveable base 114by at least a small degree. Movement of the tiller 112 relative to themoveable base 114 is restricted by centering springs 116. In theembodiment shown, the tiller sensor 118 suitably is a displacementsensor which generates an electrical signal reflecting a degree ofrotational displacement the operator has imparted to the tiller 112. Theelectrical signal generated by the tiller sensor 118 constitutes thedifferential output signal 130 from the tiller module 110. The centeringsprings 116 oppose rotational movement of the tiller 112, and thus willcause the tiller 112 to return to a neutral position relative to themoveable base 114 once the operator has released the tiller 112 or hasotherwise ceased imparting any rotational displacement to the tiller112.

Alternatively to the tiller 112 being movably mounted to the moveablebase 114, the tiller 112 suitably is rigidly mounted to the moveablebase 114 such that the tiller 112 is not moveable relative to themoveable base 114. In such an embodiment, the tiller sensors 118 i and118 ii suitably are strain gauges. Thus, instead of measuring the forceapplied to the tiller 112 by the operator as a function of thedisplacement of the tiller 112 relative to the moveable base 114, theforce is measured as a function of the strain resulting from the forceapplied to the tiller relative to the moveable base 114.

As will be further explained in connection with FIG. 3, the tillercontroller 140 receives the differential output signal 130 (or signals130 a and 130 b in a multiple tiller environment as depicted in FIG. 1)and generates a steering signal 160. The steering signal 160 is receivedat the tiller module 110 at a summer 120, whose operation will befurther described below. An output 128 of the summer 120 is received byan amplifier 122 which amplifies the signal-strength current to asuitable drive-current which is fed to a position servo motor 124. Theposition servo motor 124 in turn drives a gear head 125 which rotatesthe moveable base 114. The position of the moveable base is measured bya position sensor 126. A position output 127 of the position sensor 126is fed back to the summer 120.

At the summer 120, the position output 127 is subtracted from thesteering signal 160. The operation of the summer 120 is analogous to thefunction of the summer 180 (FIG. 1) triggering the nose gear actuationsystem 182. The steering signal 160 again reflects the desired positionof the nose gear 184. The position output 127 of the position sensor 126reflects the actual position of the moveable base 114. If the steeringsignal 160 exceeds the position signal 127, the output 128 of the summer120 will be positive. A positive output 128 from the summer 120 that isin turn input to the amplifier 122, the position servo motor 124, andthe gear head 125 rotates the moveable base 114 in what is defined as apositive direction toward the desired steering position indicated by thesteering signal 160. A positive direction, for example, may be definedas clockwise or counterclockwise as desired according to the perspectiveof FIG. 1. On the other hand, when the position signal 127 exceeds thesteering signal 160, a negative output 128 is generated by the summer120. A negative output 128 from the summer 120 that is in turn input tothe amplifier 122, the position servo motor 124, and the gear head 125rotates the moveable base 114 in what is defined as a negative directiontoward the desired steering position indicated by the steering signal160. When the steering signal 160 and the position signal 127 are equal,the signals 160 and 127 negate each other at the summer 120, therebyresulting in a zero output 128 from the summer 120. A zero output 128from the summer 120 means a zero signal is applied to the amplifier 122,the position servo motor 124, and the gear head 125, thereby causing themoveable base to 114 to remain at its current position.

In an implementation having multiple tiller modules 110 like the system100 (FIG. 1), the steering signal 160 is provided to each of the tillermodules 110. In accordance with the previously-described operation of atiller module 110, interaction of the steering signal 160 with theposition signal 127 of the position sensor of each tiller module 110results in the moveable base 114 of each tiller module 110 being movedto reflect the position of the nose gear 184. Thus, the tiller modules110 each provide feedback to their operators of the position of the nosegear 184. Again, as also previously described, use of multiple tillermodules 110 provides redundancy in case one of the tiller modules—or oneof the operators of one the tiller modules 110—should become disabled.As previously described, the operation of the tiller controller 140receives differential output signals 130 a and 130 b from multipletiller modules 110 a and 110 b, respectively, thereby allowing multipleoperators to simultaneously steer the system 100.

In a tiller module 110, redundant components or other fault tolerantstructures may be incorporated to allow the tiller module 110 to operatein the event of a failure of one or more components. For example,electrical components, such as the tiller sensor 118, summer 120,amplifier 122, and other components could be duplicated. The output ofthe components could be monitored using known fault tolerant techniquesto ensure that a reliable output signal of each component is generatedand to circumvent or replace failing components when reliable outputsignals are not produced. Mechanical components, such as the moveablebase 114 and the position servo motor 124 desirably are sufficientlyrobust so that redundant mechanical components can be omitted. However,if desired, redundant position servo motors 124 could be implemented inthe tiller module 110 with the outputs of the redundant servo motors 124supplied to a differential, for example, so that if one position servomotor 124 should fail the remaining, functional servo motor 124 couldactuate the moveable base 114. Various known fault tolerance strategiesoptionally could be used with embodiments of the tiller module 110, orembodiments of the tiller module 110 may be used without faulttolerance. In any case, the present invention is not limited toembodiments using fault tolerant devices, let alone any particular faulttolerance implementations.

A further advantage of having the tiller 112 mounted on the moveablebase 114 is for failure modes. For example, even a complete, hard-overfailure of the position servo motor 124 will not result in an erroneoussteering input signal. By virtue of the moveable base 114 and the tillercontroller 140 (FIG. 1) using a differential output signal 130 indriving the nose gear 184, occurrence of a hard-over failure will notresult in generation of a hard-over steering signal that could cause asteering mishap. In fact, failure of the position servo motor 124, initself, will not result in any steering signal at all. Nonetheless, evenif the moveable base 114 should become jammed in position as the resultof a position servo motor 124 malfunctioning, steering commands stillcould be entered using the tiller 112 because the tiller controller 140continues to receive the differential output signal resulting frompressure being applied to the tiller 112 as read relative to theposition of the movable base 114. In other words, even if the positionservo motor 124 failed to move the moveable base 114, while nose gearposition feedback would no longer be provided to the operator, theoperator could continue to steer.

FIG. 3 is a block diagram of a tiller controller 140 in accordance withan embodiment of the present invention. The differential output signals130 a and 130 b are added at a summer 142, with an output 304 of thesummer 142 supplied to a scaling factor module 306. The scaling factormodule 306 is used in an embodiment in which the tiller sensors 118(FIGS. 1 and 2) suitably are strain gauges rather than displacementsensors. When the tiller sensors 118 are strain gauges, a scaling factorcan be applied by the scaling factor module 306 to scale a magnitude ofthe signal generated by the strain gauges to represent a relativedisplacement signal between the tiller 112 and the moveable base 114. Onthe other hand, if the tiller sensors 118 are displacement sensors, thescaling factor applied by the scaling factor module 306 is unity becauseno scaling is called for. Using displacement sensors for the tillersensors 118, the scaling factor module 306 could be eliminated.

An output 308 of the scaling factor module 306 is supplied to atolerance module 310. The tolerance module 310 applies a “dead zone” tothe output 308 of the scaling factor module 306, if used, such that verysmall forces initially applied to the tiller 112 result in no change inthe steering of the system. In other words, the tolerance module 310allows for a little “play” in the tiller 112. As a result, very smallmovements of the tiller 112 caused, for example, by an operator's handtwitching as a result of involuntary muscle movements, bumps orvibrations encountered by the vehicle or other small movements, are notimmediately translated into movements of the nose gear 184.

The scaling factor module 306 and the tolerance module 310 yield a totalof the differential output signals 130 a and 130 b adjusted for the typeof tiller sensors 118 (FIG. 1) used and for slight, negligible movementsof the tiller. The resulting output 312 of the tolerance module 310 ispresented to both a proportional gain module 314 and a summer 316, anoutput 318 of which is presented to an integral gain module 320. Anoutput 362 of a position limiter module 360 and an output 372 of asignal decay module 370 are subtracted from the output 312 of thetolerance module 310 at the summer 316. Putting aside the effect of theoutputs 362 and 372, which will be described below, the proportionalgain module 314 and the integral gain module 320 applied to the combineddifferential output signals 130 a and 130 b totaled at the summer 142yields the steering signal 160. By integrating the differential outputsignals 130 a and 130 b, the steering signal 160 is proportional to theposition of the tiller 112 a and 112 b. Further, by applying theproportional output 324 of the proportional gain module 314 to thesummer 380, the rate of change of steering signal 160 is proportional tothe force applied to the tillers 112 a and 112 b. The steering signal160 is applied both to the nose gear steering actuation system 182 andthe tiller modules 110 a and 110 b and used as previously described.

The output 362 of the position limiter module 360 and the output 372 ofthe signal decay module 370, which are subtracted from the differentialoutput signals 130 a and 130 b at the summer 316, serve as limits on thesteering signal 160.

The position limiter module 360 receives an output 392 of a summer 390.The output 392 of the summer 390 is the steering signal 160 minus thenose gear position signal 188. In other words, the output 392 of thesummer 390 is the difference between the steering signal 160 and thenose gear position signal 188, which in turn is the difference betweenthe current position of the nose gear and the desired position of thenose gear. The position limiter module 360 is configured to generate theoutput 362 to limit the rate of accumulation of the integral gain module320.

More particularly, the position limiter module 360 is configured tolimit the output 322 component of the steering signal 160 to prevent thesteering signal from saturating the nose gear steering actuation system182. The more easily saturated the nose gear steering actuation system182, the greater the gain of the position limiter module 360. Thus, thesignal decay module 360 is configured empirically based on thespecifications of the nose gear steering actuation system 182.

By limiting the rate of increase of the steering signal 160, theposition limiter module 360 also limits the rate at which the moveablebases 114 of the tiller modules 110 a and 110 b will move. Limiting thesteering signal 160 limits the input to the position servo motors 124that causes the moveable bases 114 a and 114 b of the filler modules 110a and 110 b to rotate. Thus, limiting the steering signal 160 limits theability of the operators to move the tillers 112.

Ultimately, the position limiter module 360 prevents the steering signal160 and, thus, the tiller handle 112 from producing a position errorsignal 181 (FIG. 1) that significantly exceeds the rate capability ofthe steering actuation system 182. If this saturation occurs, theoperator will experience a significant increase in the opposing force atthe tiller handle 112 thereby providing tactile feedback of the nosewheel position similar to the tactile feedback provided by amechanically controlled system.

The signal decay module 370 generates an output signal 372 that servesto reduce the steering signal 160. In mechanical steering systems, whenthe operator releases the steering controls, the steering mechanismtends to return to a neutral position, such as the way that a carsteering wheel tends to rotate back to neutral when the drive lets go ofthe wheel. In a steer-by-wire system, it is desired that a signal beadded to the system to mimic the mechanical self-neutralizing effect.The steering signal 160 is the sum of both the output 322 of theintegral gain module 320 and the output 324 of the proportional gainmodule 314. Thus, even if no pressure is applied to the tillers 112 aand 112 b, the steering signal 160 will remain equal to that of theoutput 322 of the integral gain module 320. To provide for the steeringsignal 160 to decay to zero when no pressure is applied to the tillers112 a and 112 b, the signal decay module 370 applies a gain to theoutput 322 of the integral gain module 320. As long as the output 322 ofthe integral gain module 320 is nonzero, indicating that the nose gearposition is not neutral, a nonzero output 372 of the signal decay moduleis subtracted at the summer 316 to create a counter signal to offset thesteering signal 160. As in the case of the position limiter module 360,the signal decay module 370 is configured empirically with regard to thespecifications of the nose gear steering actuation system 182 to makesure that the nose gear steering actuation system 182 is not saturated.

In one exemplary embodiment of the system 100 used in a typicalcommercial aircraft, the following steering parameters may exist:

Nose gear steering range ±70 degrees Actuation system no-load rate 20degrees/second Servo valve saturation ±4 degree equivalent steeringcommand.

In such a system, the following tiller system parameters might be used:

Tiller handle range ±70 degrees Tiller handle length 4 inches Tillerhandle spring rate 2.0 pounds/degree Tiller signal gain k_(s) 1.0 ifrelative position transducer used Tiller spring rate if load transducerused Deadzone ±0.5 degrees kp  1.0 kI 10.0 Signal decay ±2.0 degreesPosition limiter ±4.5 degrees of deadzone then gain = 10 degrees/degreeFeel position servo bandpass 10 to 20 radians/second (critically damped)No-load rate 30 to 40 degrees/second Stall torque 100 to 150 inch-pounds

Using these parameters will result in the following tiller performance:

Steady-State Commanded Applied Tiller Force Steering Rate at (7) 0.0 lbs0 deg/sec (at zero tiller position) 9.0 lbs 20 deg(sec (as an increasingsteering command) 5.0 lbs 0 deg/sec (holding at a non-zero tillerposition) 0.0 lbs −20 deg/sec (tiller released at non-zero position)

In a tiller controller 140, as in the tiller module 110 (FIG. 2)redundant components or other fault tolerant structures may beincorporated to allow the tiller controller 140 to operate in the eventof a failure of one or more components. Alternatively, the entire tillercontroller 140 may be duplicated so that, if a fault is detected, thefunctions of a primary tiller controller 140 can be handled by asecondary tiller controller 140. Various known fault tolerancestrategies optionally can be used with embodiments of the tillercontroller 140, or embodiments of the tiller controller 140 may be usedwithout fault tolerance. In any case, the present invention is notlimited to embodiments using fault tolerant devices, let alone anyparticular fault tolerance implementations.

FIG. 4A is a top cutaway view of a flight deck 400 of an aircraftequipped with an embodiment of the present invention. The flight deck400 includes a captain's station 402 and a first officer's station 404.Both the captain's station 402 and the first officer's station 404 areequipped with flight controls, such as yokes 406 and 408, respectively,used to control the aircraft while in flight. Both the captain's station402 and the first officer's station 404 also are equipped with tillers410 and 412, respectively, according to an embodiment of the presentinvention that are used to steer the nose gear of the aircraft duringpush back, taxiing, and other ground operations of the aircraft. Aspreviously described, the tillers 410 and 412 are equipped to providetactile and visual feedback of the position of the nose gear. Thetillers 410 and 412 are also electrically linked to coordinate thefeedback between both tillers 410 and 412 so that an operator seated atthe captain's station 402 and an operator seated at the first officer'sstation 404 will both be apprised of the position of the nose gear asthe nose gear respond to steering commands directed by either operator.

FIG. 4B is a second top cutaway view of a flight deck 450 to show howtillers 460 and 462 are linked. As compared to the flight deck 400 (FIG.4A), in the flight deck 450 the tillers 460 and 462 have been rotatedcounterclockwise by approximately forty-five degrees as compared to thetillers 410 and 412 (FIG. 4A). Either the tiller 460 at the captain'sstation 402 and the tiller 462 at the first officer's station 404 orboth tillers 460 and 462 could be moved by the operators to thepositions shown in the flight deck 450. In any case, embodiments of thepresent invention will coordinate the movements of the tillers 460 and462. The coordinated movement of the tillers 460 and 462 providecoordinated feedback to the operators of the aircraft as to the positionof the nose gear.

FIG. 5 is a side view of an aircraft 500 that advantageously includesthe system 100 (FIG. 1). As is known, the aircraft 500 includes afuselage 510, wings 520, engines 530, control surfaces (not shown), mainlanding gear 540, and nose gear 550. The nose gear 550 are controlled bythe tillers 410 and 412 (FIG. 4A) and 460 and 462 (FIG. 4b), and theirpositions will reflect the steering position of the nose gear aspreviously described.

FIG. 6 is a flowchart of a routine 600 using an embodiment of thepresent invention operating with a single tiller module. The routine 600begins at a block 602 when the tiller handle is engaged by an operator.At a block 604 the tiller sensor reads actuation of tiller handlerelative to the moveable base. As previously described, the tillersensor can be a displacement sensor or a strain gauge. At a block 606 arelative displacement signal is generated, either directly from adisplacement sensor or from a scaling factor module in response to astrain gauge reading. At a block 608, using methods previously describedin connection with FIG. 3, the relative displacement signal is convertedto a steering signal. At a block 610 the steering signal is supplied tothe steering mechanism. At a block 612 the steering signal is alsosupplied to reposition the base of the tiller to reflect the currentsteering position, thereby providing operator feedback in thesteer-by-wire system. At a decision block 614 it is determined if forcecontinues to be applied to the tiller handle by the operator. If so, theroutine 600 loops to the block 604 where the tiller sensor reads theactuation of the tiller handle relative to the base. If not, at a block616 the steering mechanism and the tiller base are returned to thestraight-ahead position according to the signal decay coefficient. Oncethe steering mechanism and the tiller base are returned to thestraight-ahead position, the routine 600 ends at a block 620.

FIG. 7 is a flowchart of a routine 700 using an embodiment of thepresent invention operating with two tiller modules. It will beappreciated that the routine 700 could be adapted to any number oftiller modules. The routine 700 begins at a block 702 when a tillerhandle is engaged by one of the operators. At a block 704 a tillersensor reads actuation of tiller handle A relative to moveable base Aand, at the same time, at a block 706 a tiller sensor reads actuation oftiller handle B relative to moveable base B. At a block 708 a relativedisplacement signal of the tiller A relative to the base is generatedand, at the same time, at a block 710 a relative displacement signal ofthe tiller B relative to the base is generated. At a block 712 therelative displacement signals of tiller A and tiller B are summed. At ablock 714 the summed displacement signal is converted to a steeringsignal. At a block 716 the steering signal is supplied to the steeringmechanism. At a block 718 the steering signal is also supplied toreposition the bases of tiller A and tiller B to reflect the currentsteering angle, thereby providing feedback to each of the operators inthe steer-by-wire system. At a decision block 720 it is determined ifforce continues to be applied to the tiller handle by either of theoperators. If so, the routine 700 loops to the blocks 704 and 706 wherethe tiller sensor reads the actuation of the tiller handles relative tothe bases. If not, at a block 722 the steering mechanism and the tillerbases are returned to the straight ahead position according to thesignal decay coefficient. Once the steering mechanism and the tillerbases are returned to the straight ahead position, the routine 700 endsat a block 724.

While the preferred embodiment of the invention has been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

What is claimed is:
 1. A system for generating a signal to control asteering mechanism, the system comprising: a tiller module configured toreceive a tiller, the tiller module including: a moveable tiller baseconfigured to moveably respond to a steering signal; and a tillercoupling configured to couple the tiller with the moveable tiller baseand further configured to generate a tiller differential signalindicative of a steering force applied to the tiller relative to aposition of the moveable tiller base; and a tiller controller configuredto receive the tiller differential signal and a steering mechanismposition signal, the tiller controller being further configured togenerate the steering signal to direct a steering mechanism to conformwith the steering force and to direct the moveable tiller base to afeedback position representative of a current position of the steeringmechanism.
 2. The system of claim 1, wherein the tiller couplingincludes a centering spring mechanism configured to receive the tillerand allow movement of the tiller relative to the moveable tiller basewithin a predetermined displacement range.
 3. The system of claim 2,wherein the tiller input coupling includes a tiller position transducerconfigured to measure a displacement of the tiller and generate thetiller differential signal.
 4. The system of claim 1, wherein the tillerinput coupling includes a force transducer configured to generate thetiller differential signal as a function of a strain resulting from thesteering force applied to the tiller relative to the position of themoveable tiller base.
 5. The system of claim 1, wherein the tillerincludes a rotatable handle.
 6. The system of claim 5, wherein themoveable tiller base includes a rotatable base moveable by a motorreceiving the steering signal.
 7. The system of claim 1, wherein thetiller controller converts the tiller differential signal into thesteering signal such that the moveable tiller base is moved to cause thetiller to be aligned to a position representative of the steeringmechanism position signal.
 8. The system of claim 7, wherein the tillercontroller integrates the tiller differential signal such that a rate ofchange of the steering signal is proportional to a magnitude of thesteering force.
 9. The system of claim 1, wherein the tiller controllerfurther includes a steering signal decay function such that a reductionin the tiller differential signal causes a resulting reduction in thesteering signal causing the steering mechanism and the moveable tillerbase to return toward a neutral position.
 10. The system of claim 1,wherein the tiller controller further includes a position limiterrestricting the tiller such that the feedback position of the tiller isrepresentative of the current position of the steering mechanism. 11.The system of claim 1, wherein the tiller module further comprises aplurality of tiller module components including at least one of a servomotor, a gear head, an amplifier, and a position sensor.
 12. The systemof claim 11, wherein the tiller module farther includes a plurality ofredundant tiller module components operable to duplicate functions of atleast some of the tiller module components.
 13. The system of claim 1,wherein the tiller controller further comprises a plurality of tillercontroller components including at least one of a summer, an integrator,and a gain device.
 14. The system of claim 13, wherein the tillercontroller further includes a plurality of redundant tiller controllercomponents operable to duplicate functions of at least some of thetiller controller components.
 15. The system of claim 1, wherein thesystem includes a secondary tiller controller operable to replacefunctions of the tiller controller.
 16. The system of claim 1, whereinthe tiller module includes: a first tiller module having a firstmoveable tiller base configured to receive a first tiller that isconfigured to receive a first steering force; and a second tiller modulehaving a second moveable tiller base configured to receive a secondtiller that is configured to receive a second steering force.
 17. Thesystem of claim 16, wherein the first tiller module generates a firsttiller differential signal and the second tiller module generates asecond tiller differential signal, the tiller controller summing thefirst tiller differential signal and the second tiller differentialsignal such that the steering signal is generated to direct the steeringmechanism to conform with a composite of the first steering force andthe second steering force.
 18. The system of claim 17, wherein thesteering signal causes both the first moveable tiller base and thesecond moveable tiller base to the feedback position representative ofthe current position of the steering mechanism.
 19. A system forgenerating a signal to control a steering mechanism, the systemcomprising: a plurality of tiller modules, each of the tiller modulesincluding: a moveable tiller base configured to moveably respond to asteering signal; and a tiller coupling configured to receive a tillerand couple the tiller with the moveable tiller base and furtherconfigured to generate a tiller differential signal reflecting asteering force applied to the tiller relative to a position of themoveable tiller base; and a composite tiller controller including: areceiver configured to receive each of the tiller differential signalsand a steering mechanism position signal; a summing device configured tosum the tiller differential signals to generate a composite tillerdifferential signal to represent a composite steering force of aplurality of steering forces; and a signal generator configured togenerate the steering signal to direct the steering mechanism to conformwith the composite steering force and direct each of the moveable tillerbases to a feedback position representative of a current position of thesteering mechanism.
 20. The system of claim 19, wherein at least one ofthe tiller modules includes a centering spring mechanism configured toreceive the tiller and allow movement of the tiller relative to themoveable tiller base within a predetermined displacement range.
 21. Thesystem of claim 20, wherein the tiller input coupling includes a tillerposition transducer configured to measure a displacement of the tillerand generate the tiller differential signal.
 22. The system of claim 19,wherein the tiller input coupling includes a force transducer configuredto generate the tiller differential signal as a function of a strainresulting from the steering force applied to the tiller relative to theposition of the moveable tiller base.
 23. The system of claim 19,wherein the tiller includes a rotatable handle.
 24. The system of claim23, wherein the moveable tiller base includes a rotatable base moveableby a motor receiving the steering signal.
 25. The system of claim 19,wherein the tiller controller converts the composite tiller differentialsignal into the steering signal such that the moveable tiller base ismoved to cause the tiller to be aligned to a position representative ofthe steering mechanism position signal.
 26. The system of claim 25,wherein the tiller controller integrates the composite tillerdifferential signal such that a rate of change of the steering signal isproportional to a magnitude of the composite steering force.
 27. Thesystem of claim 19, wherein the tiller controller further includes asteering signal decay function such that a reduction in the tillerdifferential signal causes a resulting reduction in the steering signalcausing the steering mechanism and the moveable tiller base to returntoward a neutral position.
 28. The system of claim 19, wherein thetiller controller further includes a position limiter restricting thetiller such that the feedback position of the tiller is representativeof the current position of the steering mechanism.
 29. The system ofclaim 19, wherein the tiller module further comprises a plurality oftiller module components including at least one of a servo motor, a gearhead, an amplifier, and a position sensor.
 30. The system of claim 29,wherein the tiller module further includes a plurality of redundanttiller module components operable to duplicate functions of at leastsome of the tiller module components.
 31. The system of claim 19,wherein the tiller controller further comprises a plurality of tillercontroller components including at least one of a summer, an integrator,and a gain device.
 32. The system of claim 31, wherein the tillercontroller further includes a plurality of redundant tiller controllercomponents operable to duplicate functions of at least some of thetiller controller components.
 33. The system of claim 19, wherein thesystem includes a secondary tiller controller operable to replacefunctions of the tiller controller.
 34. An aircraft comprising: afuselage; at least one engine; at least one wing; a plurality of controlsurfaces; main landing gear; nose gear; and a system for generating asignal to control a steering mechanism, the system comprising: a tillermodule configured to receive a tiller, the tiller module including: amoveable tiller base configured to moveably respond to a steeringsignal; and a tiller coupling configured to couple the tiller with themoveable tiller base and further configured to generate a tillerdifferential signal indicative of a steering force applied to the tillerrelative to a position of the moveable tiller base; and a tillercontroller configured to receive the tiller differential signal and asteering mechanism position signal, the tiller controller being furtherconfigured to generate the steering signal to direct a steeringmechanism to conform with the steering force and to direct the moveabletiller base to a feedback position representative of a current positionof the steering mechanism.
 35. The aircraft of claim 34, wherein thetiller coupling includes a centering spring mechanism configured toreceive the tiller and allow movement of the tiller relative to themoveable tiller base within a predetermined displacement range.
 36. Theaircraft of claim 35, wherein the tiller input coupling includes atiller position transducer configured to measure a displacement of thetiller and generate the tiller differential signal.
 37. The aircraft ofclaim 34, wherein the tiller input coupling includes a force transducerconfigured to generate the tiller differential signal as a function of astrain resulting from the steering force applied to the tiller relativeto the position of the moveable tiller base.
 38. The aircraft of claim34, wherein the tiller includes a rotatable handle.
 39. The aircraft ofclaim 38, wherein the moveable tiller base includes a rotatable basemoveable by a motor receiving the steering signal.
 40. The aircraft ofclaim 34, wherein the tiller controller converts the tiller differentialsignal into the steering signal such that the moveable tiller base ismoved to cause the tiller to be aligned to a position representative ofthe steering mechanism position signal.
 41. The aircraft of claim 40,wherein the tiller controller integrates the tiller differential signalsuch that a rate of change of the steering signal is proportional to amagnitude of the steering force.
 42. The aircraft of claim 34, whereinthe tiller controller further includes a steering signal decay functionsuch that a reduction in the tiller differential signal causes aresulting reduction in the steering signal causing the steeringmechanism and the moveable tiller base to return toward a neutralposition.
 43. The aircraft of claim 34, wherein the tiller controllerfurther includes a position limiter restricting the tiller such that thefeedback position of the tiller is representative of the currentposition of the steering mechanism.
 44. The aircraft of claim 34,wherein the tiller module further comprises a plurality of tiller modulecomponents including at least one of a servo motor, a gear head, anamplifier, and a position sensor.
 45. The aircraft of claim 44, whereinthe tiller module further includes a plurality of redundant tillermodule components operable to duplicate functions of at least some ofthe tiller module components.
 46. The aircraft of claim 34, wherein thetiller controller further comprises a plurality of tiller controllercomponents including at least one of a summer, an integrator, and a gaindevice.
 47. The aircraft of claim 46, wherein the tiller controllerfurther includes a plurality of redundant tiller controller componentsoperable to duplicate functions of at least some of the tillercontroller components.
 48. The aircraft of claim 34, wherein the systemincludes a secondary tiller controller operable to replace functions ofthe tiller controller.
 49. The aircraft of claim 34, wherein the tillermodule includes: a first tiller module having a first moveable tillerbase configured to receive a first tiller that is configured to receivea first steering force; and a second tiller module having a secondmoveable tiller base configured to receive a second tiller that isconfigured to receive a second steering force.
 50. The aircraft of claim49, wherein the first tiller module generates a first tillerdifferential signal and the second tiller module generates a secondtiller differential signal, the tiller controller summing the firsttiller differential signal and the second tiller differential signalsuch that the steering signal is generated to direct the steeringmechanism to conform with a composite of the first steering force andthe second steering force.
 51. The aircraft of claim 50, wherein thesteering signal causes both the first moveable tiller base and thesecond moveable tiller base to the feedback position representative ofthe current position of the steering mechanism.
 52. A method forgenerating a signal to control a steering mechanism, the methodcomprising: moving a moveable tiller base in response to a steeringsignal; generating a tiller differential signal reflecting a steeringforce applied to a tiller relative to a position of the moveable tillerbase; and generating the steering signal by comparing the tillerdifferential signal with a steering mechanism position signal, directingthe steering mechanism to conform with the steering force, and directingthe moveable tiller base to a feedback position representative of acurrent position of the steering mechanism.
 53. The method of claim 52,further comprising allowing movement of the tiller relative to themoveable tiller base within a predetermined displacement range.
 54. Themethod of claim 53, further comprising measuring the tiller differentialsignal by measuring a displacement of the tiller relative to themoveable tiller base.
 55. The method of claim 52, wherein the tillerdifferential signal is measured as a function of a strain resulting fromthe steering force applied to the tiller relative to the position of themoveable tiller base.
 56. The method of claim 52, wherein the tillerincludes a rotatable handle.
 57. The method of claim 56, wherein a motormoves the rotatable base in response to receiving the steering signal.58. The method of claim 57, further comprising converting the tillerdifferential signal into the steering signal such that the moveabletiller base is moved to cause the tiller to be aligned to a positionrepresentative of the steering mechanism position signal.
 59. The methodof claim 58, further comprising integrating the tiller differentialsignal such that a rate of change of the steering signal is proportionalto a magnitude of the steering force.
 60. The method of claim 52,further comprising applying a steering signal decay function such that areduction in the tiller differential signal causes a resulting reductionin the steering signal, thereby causing the steering mechanism and themoveable tiller base to return toward a neutral position.
 61. The methodof claim 52, further comprising applying a position limiter restrictingmovement of the tiller such that the feedback position of the tiller isrepresentative of the current position of the steering mechanism. 62.The method of claim 52, further comprising providing a plurality oftiller module components configured to move the moveable tiller base inresponse to the steering signal and generate the tiller differentialsignal, the plurality of tiller module components including at least oneof a servo motor, a gear head, an amplifier, and a position sensor. 63.The method of claim 62, further comprising providing a plurality ofredundant tiller module components operable to duplicate functions of atleast some of the tiller module components.
 64. The method of claim 52,further comprising providing a plurality of tiller controller componentsconfigured to generate the steering signal, the tiller controllercomponents including at least one of a summer, an integrator, and a gaindevice.
 65. The method of claim 64, further comprising providing aplurality of redundant tiller controller components operable toduplicate functions of at least some of the tiller controllercomponents.
 66. The method of claim 52, further comprising providing asecondary tiller controller operable to replace functions of the tillercontroller.
 67. The method of claim 52, further comprising receiving afirst tiller configured to receive a first steering force into a firstmoveable tiller base and receiving a second tiller configured to receivea second steering force into a second moveable tiller base.
 68. Themethod of claim 67, further comprising generating a first tillerdifferential signal indicative of a first steering force applied to thefirst tiller and generating the second tiller differential signalindicative of a second steering force applied to a second tiller, suchthat the steering signal is generated to direct the steering mechanismto conform with a composite of the first steering force and the secondsteering force.
 69. The method of claim 68, wherein the steering signalcauses both the first moveable tiller base aid the second moveabletiller base to move to the feedback position representative of thecurrent position of the steering mechanism.
 70. A method for generatinga signal to control a steering mechanism, the method comprising: movinga plurality of moveable tiller bases in response to a plurality ofsteering signals; generating a plurality of tiller differential signalsreflecting a plurality of steering forces applied to a plurality oftillers relative to positions of the plurality of respective moveabletiller bases; and generating the plurality of steering signals bysumming the plurality of tiller differential signals to generate acomposite tiller differential signal; comparing the composite tillerdifferential signal and a steering mechanism position signal; directingthe steering mechanism to conform with a composite steering force of theplurality of steering forces; and directing the plurality of moveabletiller bases to a feedback position representative of a current positionof the steering mechanism.
 71. The method of claim 70, furthercomprising allowing movement of the plurality of tillers relative to theplurality of respective moveable tiller bases within a predetermineddisplacement range.
 72. The method of claim 71, further comprisingmeasuring each of the plurality of tiller differential signals bymeasuring displacements of the plurality of tillers relative to theplurality of the respective moveable tiller bases.
 73. The method ofclaim 70, wherein each of the plurality of tiller differential signalsis measured as a function of stains resulting from the plurality ofsteering forces applied to the tillers relative to the positions of theplurality of respective moveable tiller bases.
 74. The method of claim70, wherein each of the plurality of tillers includes a rotatablehandle.
 75. The method of claim 74, wherein a motor moves each of theplurality of the respective rotatable bases in response to receiving thesteering signal.
 76. The method of claim 70, further comprisingconverting the plurality of the tiller differential signals into thesteering signal such that each of the plurality of the moveable tillerbases is moved to cause each of the plurality of tillers to be alignedto a position representative of the steering mechanism position signal.77. The method of claim 76, further comprising integrating the pluralityof tiller differential signals such that a rate of change of thesteering signal is proportional to a magnitude of the composite steeringforce.
 78. The method of claim 70, further comprising applying asteering signal decay function such that a reduction in the tillerdifferential signal causes a resulting reduction in the steering signalcausing the steering mechanism and the plurality of moveable tillerbases to return toward a neutral position.
 79. The method of claim 70,further comprising applying a position limiter restricting movement ofthe plurality of tillers such that the feedback position of theplurality of tillers is representative of the current position of thesteering mechanism.
 80. The method of claim 70, further comprisingproviding a plurality of tiller module components configured to move themoveable tiller base in response to the steering signal and generate thetiller differential signal, the plurality of tiller module componentsincluding at least one of a servo motor, a gear head, an amplifier, anda position sensor.
 81. The method of claim 80, further comprisingproviding a plurality of redundant tiller module components operable toduplicate functions of at least some of the tiller module components.82. The method of claim 70, further comprising providing a plurality oftiller controller components configured to generate the steering signal,the tiller controller components including at least one of a summer, anintegrator, and a gain device.
 83. The method of claim 82, furthercomprising providing a plurality of redundant tiller controllercomponents operable to duplicate functions of at least some of thetiller controller components.
 84. The method of claim 70, furthercomprising providing a secondary tiller controller operable to replacefunctions of the tiller controller.